1. Field of the Invention
The present invention relates to a robot system controlling method and robot system which perform link angle control and stiffness control. Suitably, the present invention is applied to a working robot and a legged locomotion robot.
2. Description of the Related Art
In manipulator controlling methods, it has become important that end effectors (such as hands) installed at tips of links can flexibly touch an object. If this is applied to an industrial robot, human-robot collaboration can be realized. Then, by controlling the direction of flexibility of the hands, part-fitting operations will be made easier. On the other hand, if this is applied to a legged locomotion robot, shocks to the body can be cushioned as the robot touches the ground softly. Also, the robot will be able to walk stably on an uneven ground surface by absorbing level differences.
To control flexibility of hands, available methods include impedance control which involves attaching force sensors to hands and control which uses artificial muscle actuators. It is known that human muscles are variable viscoelastic control mechanisms as well as actuators. Among artificial muscles, pneumatic rubber artificial muscles typified by McKibben artificial muscles are especially similar to human muscles in viscoelastic properties. Then, using softness control of the artificial muscle actuators installed in manipulators, the hands can be caused to touch an object with desired flexibility. However, the artificial muscle actuators are known to have a problem in controllability because the artificial muscle actuators have nonlinear viscoelastic properties and the artificial muscle actuators, which generate forces only in a contraction direction, must be controlled via antagonistic arrangement.
Due to this problem, Japanese Patent Application Laid-Open No. S63-68382 discloses a technique for placing a rubber artificial muscle actuator supplied with a compressed fluid in antagonism to a link, detecting an angle at a joint, and thereby controlling a swing angle of the link at the joint through feedback control.
Also, Japanese Patent No. 3436320 discloses a technique for generating a manipulator model including viscoelastic properties of muscles as well as feed forward input used to simultaneously control a joint angle and hand flexibility using a correction calculation unit. Upon receiving a control input, the technique disclosed in Japanese Patent No. 3436320 outputs the joint angle and a viscoelastic coefficient of an artificial muscle actuator using the model and compares the joint angle and viscoelastic coefficient with target values. Then, departures from the target values are propagated back to the correction calculation unit to correct feed forward input. The corrected feed forward input is given to the model again. This process is repeated to refine the feed forward input.
However, although the technique disclosed in Japanese Patent Application Laid-Open No. S63-68382 detects the swing angle of the link and performs feedback control to attain a target swing angle, the technique controls only the swing angle of the link at the joint, and does not realize control intended to vary the flexibility (in other words, stiffness) of the link at the joint.
Furthermore, the technique disclosed in Japanese Patent No. 3436320 simultaneously controls the swing angle of a link and flexibility of the link at the joint through feed forward control. Inputs for feed forward control are determined by correcting departures from the target values repeatedly, and thus very large amounts of computation are required in order to generate the inputs. Consequently, iterative calculations need to be performed each time the target values for the swing angle of the link and flexibility of the joint interconnecting the links are changed. Thus, the technique has a problem in that time is required for calculations when the target values are changed.
The problem occurs not only when an artificial muscle actuator is used as a driving unit adapted to drive the link, but also when a rotating motor is used as a driving unit for control.
Thus, an object of the present invention is to provide a robot system controlling method and robot system which perform link angle control and joint stiffness control through feedback control.